Data transmissions in a wireless communications system may be scheduled with transmissions from multiple user devices to a network node. This may be achieved through the exchange of control messages to schedule the transmissions over shared resources in the network. Data transmissions also may be unscheduled in order to reduce latency and overhead. Different data encoding techniques, such as Sparse Code Multiple Access (SCMA), have been proposed as improvements over traditional methods e.g. Code Division Multiple Access (CDMA). In SCMA systems, binary data streams are encoded directly to multi-dimensional codewords to spread data over multiple resource elements. Sparse codewords may be used for scheduled or unscheduled transmissions and may be decoded using message passing algorithms (MPAs).